The present invention generally relates to synchronizing signal generating apparatuses, and more particularly to a synchronizing signal generating apparatus capable of generating synchronizing signals having different frequencies which are used in different television systems, by use of the same circuit construction.
Presently, television systems employed throughout the world are not the same, and numerous different television systems exist each each part of the world. For example, in black-and-white television systems, there are television systems designated by characters such as "A", "B", "C", "D", "E", "G", "H", "I", "K", "K1", "L", "M", "N", and the like before the same of the system. The number of scanning lines is 2405 in the system A, while the number of scanning lines is 819 in the system E, but these systems are used less frequently these days. Among the above systems excluding the systems A and E, only the system M employs 525 scanning lines, and the horizontal scanning frequency is 15.75 kHz (in the case of a color television system which will be described hereinafter, the horizontal scanning frequency is 15.734 . . . kHz). All the other systems employ 625 scanning lines, and the horizontal scanning frequency is 15.625 kHz.
On the other hand, among color television signals, there are three types, that is, the "NTSC", "PAL", and "SECAM" system color television signals according to the transmission format of the chrominance signal. There are several color television systems which combine one of these three types of color television signals with one of the above described television systems. In terms of the relation between the chrominance subcarrier frequency f.sub.SC and the horizontal scanning frequency f.sub.H, the color television systems may be generally classified into five systems, that is, M/NTSC system, M/PAL system, B, G, H, or I/PAL system (hereinafter simply referred to as a B/PAL system), N/PAL system, and B, D, G, H, K1, or L/SECAM system (hereinafter simply referred to as a SECAM system).
For example, because the chrominance subcarrier frequency f.sub.SC of the
system color television system comprises offsets of f.sub.H /4 and f.sub.V /2, where f.sub.H is the horizontal scanning frequency of the PAL system and f.sub.V is the vertical scanning frequency of the PAL system, the following equation stands between the frequencies f.sub.SC and f.sub.H. EQU f.sub.SC =(284-1/4+1/625)f.sub.H =283.7516f.sub.H
Hence, the frequencies f.sub.SC and f.sub.H cannot be described by a simple ratio of integers.
The following relations are obtained between the chrominance subcarrier frequency f.sub.SC and the horizontal scanning frequency f.sub.H for the five color television systems. ##EQU1##
As clearly seen from the above equations (1) through (4), the relation between the chrominance subcarrier frequency f.sub.SC and the horizontal scanning frequency f.sub.H cannot be described by a simple ratio of integers for the four color television systems excluding the SECAM system, and in these four color television systems, it was difficult to obtain the horizontal scanning frequency f.sub.H by frequency-dividing the chrominance subcarrier frequency f.sub.SC. Accordingly, the conventional synchronizing signal generating apparatus was designed to obtain sums and differences of frequencies by use of a balanced oscillator and the like. As a result, the circuit construction of the conventional synchronizing signal generating apparatus become complex, and analog circuits coexisted with digital circuits. Further, due to reasons such as the need for precise adjustment of the analog circuits and the like, it was difficult to design the circuit as a single integrated circuit (IC). In addition, the conventional synchronizing signal generating circuit merely generated a synchronizing signal of one of the above five color television systems.